VELOCITY— ANNUAL VAKIATION. 



97 



an hour during March, 1903, at Snow Hill in the west Antarctic* The highest velocity of the 

 two years occurred on May i, 1912, when a wind of 81 miles in the hour was recorded. 



Annual Variation of Wind Velocity. 



Examining the numbers in the last column of table 55 which give the mean monthly 

 values for the four years (plotted on figure 31), we see that the annual variation has a 

 double period with two maxima and two minima. It would probably be more correct to say 



Fig. 31. Annual variation of wind velocity. 



that the annual variation has a main period with a minimum in the summer and a 

 maximum in the winter upon which is superposed a pronounced secondary maximum in 

 February and March. 



With large variations in velocity from year to year which we have already noted it ia 

 not surprising to find that the annual variations are not identical from year to year. Thus 

 the absolute maximum of monthly wind velocity occurred in May in 1902, in October in 

 1903, in March in 1911, and in June in 1912. 



Nevertheless by examining the whole trend of each curve we see that the wind in each 

 year does conform more or le.ss closely to the mean of the four years, i.e., in each year 

 there is a marked minimum in January, a maximum in February or March and then a second 

 maximum in one or other of the winter months. We may therefore accept the mean of 

 the four yeats as representing fairly closely the normal annual variation of wind strength. 



The physical explanation of this annual variation would be interesting, but it cannot be 

 given with any certainty. The following considerations may however be useful in indicating 

 the lines along which an explanation may be found. 



* Since this was written Mawson haa reported the tremendou-s wind velocities found in Adelie Land, where 

 the mean velocity for a year was 50 miles an hour. 



13 



